1. Field of the Invention
The present invention relates to an interconnection structure for a high-speed electrical interconnection between a differential transmission line and a single-ended line and, more particularly, to a differential pair interconnection apparatus for providing differential signals on a printed circuit board (PCB) having signal paths for high-speed differential signals to an external circuit, or for providing signals inputted from the external circuit to the printed circuit board, without any signal distortion.
2. Discussion of Related Art
As technology advances, various types of integrated circuits (ICs) have been developed, and thus, an operating speed of the ICs has been also gradually increased. Recently, an IC capable of transmitting data with a data transfer rate in the range of 5 to 10 Gbps, or more, has been developed. However, in spite of such a technology, the data transfer rate may be limited by various factors, which may be generated in a signal transmission path. As for the major factors that confine the data transfer rate, there may be impedance mismatching and cross talk. Accordingly, as one method for solving such problems, a differential signal line operating by means of differential signals has been introduced. The differential signal line is comprised of two conductive paths located adjacent to each other, and the signals thereof are processed with a voltage difference between the two conductive paths. Since the two conductive paths have the same amount of electrical noises that may electromagnetically affect the differential signal line, common noise voltages generated in the two conductive paths does not affect the signals. Therefore, the differential signal line is less sensitive to cross talk than the single-ended signal line.
However, despite of employing the differential signal line, discontinuity and impedance mismatching in various interconnection structures have influenced on the signal transmission. In a general interconnection structure which transmits signals from one circuit board to the other circuit board or module, if a signal frequency increases, various problems may be occurred such as increases of parasitic components, impedance mismatching and so on. As one of representative conventional technologies related to the interconnection structure, there has been PCT Patent Publication No. WO2001/39332 (by TERADYNE Inc., published May 31, 2001). This International Patent provides a modular electrical connector that provides signal paths for differential signals between a motherboard and a daughter board or other electrical components. The electrical connector transmits a plurality of differential signals and includes a plurality of pair signal conductors with two signal paths, and the plurality of pair signal conductors may control cross talk by operating with a differential mode. Accordingly, the modular electrical connector is attached to a printed circuit board such as a motherboard, daughter board, or backplane, and has a configuration including differential signal lines on the printed circuit board.
As another interconnection structure, there has been a high speed and density interconnection system for differential and single-ended transmission applications designed by NORTHROP GRUMMAN CORPORATION (U.S. Provisional Patent Application Ser. No. 60/328,396, filed Oct. 12, 2001). The high density interconnect system has a configuration including a built-in coaxial cable that contains two inner conducting wires rather than one, and the two inner conducting wires provide two physical channels. This system has a latching mechanism for compression mount type electrical connectors, and provides a high speed and density electrical connector having a central twinax or coax portion and fuzz button contacts on opposite ends of the central twinax or coax portion.
As still another interconnection structure, there is a connector for interconnection between a strip line and a coaxial cable, provided by HEWLETT PACKARD Co. (U.S. Pat. No. 5,404,117, issued on Apr. 4, 1995). The patent provides an optimal configuration in a transition area so as to minimize a reflection problem and signal distortion in the transition area, which may be caused by signal transition from the coax cable to the strip line. If a frequency of a transited signal reaches several GHz or more and a signal rising time (tr) becomes pico-seconds, the signal is greatly distorted in an abrupt transition area. Accordingly, to solve this problem, a configuration having a cosine form has been proposed.
FIG. 1 shows an interconnection structure of conventional single-ended lines and SMA connectors.
Differential signal lines 101a and 101b are formed on a printed circuit board (PCB) 100 so as to transmit signals with no changes in differential impedance. The differential signal lines 101a and 101b are connected to SMA connectors 201a and 201b, respectively, for an interconnection with the general single-ended signal lines connected to an external circuit and an apparatus. The SMA connectors 201a and 201b are supported and fixed on the circuit board 100 by housings 202a and 202b, respectively, and central pins 203a and 203b are connected to the differential signal lines 101a and 101b on the circuit board 100 through the housings 202a and 202b. 
The differential signal lines 101a and 101b should be placed closer each other for an interaction. However, since the aforementioned configuration does not have an enough space to interconnect with the SMA connectors 201a and 201b, the differential signal lines 101a and 101b must inevitably be separated by a predetermined distance or more, in order to interconnect the differential signal lines 101a and 101b to the SMA connectors 201a and 201b, whereby a bending of the differential signal lines 101a and 101b comes to be required. An isolation of the differential signal lines 101a and 101b causes a weakening of a cohesive force between signals, and thus, the signals are transmitted from a differential mode to the single-ended mode and also benefits obtained by the differential signal lines 101a and 101b cannot be retained anymore due to bending areas 102 and 103. Therefore, in order to minimize an influence caused by line widths or bending of the differential signal lines 101a and 101b, chamfered bend configurations 102 and 103, which cut certain areas of the differential signal lines 101a and 10b may be formed. However, in spite of such efforts, the differential signal lines 101a and 101b operate with the single-ended mode, so that discontinuity or impedance may occur. As a result, distortion of the signals occurs. Thus, for this reason, it is difficult to design and manufacture an interconnection apparatus capable of transmitting signals at a high speed of 5 Gbps or more.